Helicity Removal and Coronal Fe XII Stalks: Evidence That the Axial Field Is Not Ejected but Resubmerged

Abstract

The magnetic/current helicity of the coronal field is closely associated with the presence of a nonpotential axial component directed along the photospheric polarity inversion line (PIL), which is also the source of the axial/toroidal field in flux ropes and coronal mass ejections (CMEs). To better understand the role of this axial component in the evolution of coronal helicity, we use Fe xii 19.3 nm images and longitudinal magnetograms from the Solar Dynamics Observatory to track active regions (ARs) and their filament channels as they decay due to flux transport processes. We find that the Fe xii loop legs or "stalks," initially oriented almost perpendicular to the PIL, become closely aligned with it after ~1–4 rotations; this alignment is attributed to the progressive cancellation of the transverse field component at the PIL. As the AR flux continues to decay, the PIL becomes ever more distorted and the directions of the stalks are increasingly randomized. These observations suggest that most of the original axial field in ARs is not expelled in CMEs, but instead pinches off after the eruptions and becomes concentrated at the PIL. Because the twist of the field decreases, however, the helicity itself decreases, with CMEs removing a significant fraction of it in the form of disconnected flux ropes. Like most of the AR flux, the bulk of the axial field is eventually canceled/resubmerged, brought to the equator by the subsurface meridional flow, and annihilated (along with the remaining helicity) by merging with its opposite-handed counterpart from the other hemisphere.